1. Field of the Invention
The present invention relates to a thermostat that opens and closes a cooling water passage, a water cooling device including the thermostat, a water-cooled engine including the water cooling device, and a vessel propulsion apparatus powered by the engine.
2. Description of the Related Art
Japanese Patent Application Publication No. 2008-95918 discloses a thermostat disposed inside a cooling water passage defined by two side walls. The thermostat includes a thermo element that moves a valve, a frame that supports the thermo element, and a rubber body sandwiched in the axial directions by the two side walls.
As shown in FIG. 5 of this publication, the set value of the gap in the radial directions between the frame of the thermostat and the inner wall surface of the cooling water passage is small. In this case, with a variation of the fitting position of the thermostat, or a positional change of the thermostat caused by aging or deterioration, the frame of the thermostat may come into contact with the inner wall surface of the cooling water passage due to factors such as variations in the dimensions of the thermostat.
A conventional frame is made of copper or stainless steel, and a conventional cooling water passage is made of aluminum. Specifically, the cooling water passage is made of a metal material having an ionization tendency higher than that of the frame. Therefore, if the frame is in continuous contact with the inner wall surface of the cooling water passage and cooling water remains between the frame and the cooling water passage, corrosion of the cooling water passage progresses, and a hole that causes water leakage may be formed. In particular, when the cooling water is seawater, the cooling water passage corrodes more quickly.
To prevent the frame of the thermostat and the inner wall surface of the cooling water passage from coming into contact with each other, the gap in the radial directions between the frame and the cooling water passage may be increased. However, if the gap between the frame and the cooling water passage is large, an allowable movement amount of the frame with respect to the inner wall surface of the cooling water passage is decreased, so that when the thermostat is fitted to the cooling water passage, the stability of the position of the thermostat (before the sealing member of the thermostat is inserted) is deteriorated. Therefore, the fixing position (position after the sealing member is inserted) of the thermostat may deviate from an intended position. Therefore, even if the gap between the frame and the cooling water passage is increased, a gap having an intended size may not be achieved between the frame and the cooling water passage.